Many modern vehicles, in particular sport utility vehicles and light trucks, are equipped with a four-wheel drive system to improve traction, which comprises a transfer case for interconnecting and also distributing power between the primary and secondary drivelines to cause the vehicle to drive in a four-wheel drive condition. Such four-wheel drive vehicles are equipped with a transfer case, which receives drive torque from the transmission of the vehicle and connects or distributes the power between the primary driveline for driving a first set of wheels (for example, the rear wheels) and the secondary driveline for driving a second set of wheels (for example, the front wheels).
Depending on the particular four wheel drive systems, the engagement of four wheel drive mode may be manually controlled by selection of the four-wheel drive mode by the operator, or it can be automatically controlled by an electronic control system upon detection of adverse road conditions, such as a low traction condition when driving on slippery road surfaces or when driving off road.
Certain transfer cases used for manual shift drive systems or automatic control drive systems (such as “on-demand” control systems) use a multiple plate clutch to transmit torque. The clutch is actuated by a mechanical, magnetic, or hydraulic mechanism.
In a conventional transfer case, such as those equipped with manual four wheel drive selection, the front wheels are typically engaged with a synchronizer that has splined teeth to transmit the drive torque. The synchronizer is usually moved into engagement by an electric motor. This design has some merits since the system can be made with relatively low cost.
However, one primary drawback recognized with this design is the possibility of malfunction or inability in releasing from the four-wheel drive engagement, particularly when the system is subject to torque load. The synchronizer spline teeth will engage with no torque load, but friction on the spline teeth will often prevent them from disengagement if they are transmitting high torque. Thus, even if the driver selects the two-wheel drive mode, the synchronizer often remains in a locked condition of the four-wheel drive mode. If the secondary wheels (e.g., the front wheels) remain engaged when driving on regular roads, the difference between front and rear wheel speeds will produce destructive torque in the driveline of the vehicle. This stress may build up in the driveline until the system has a torque reversal that will let the synchronizer release, or otherwise some parts in the driveline may be damaged or break.